Imagine that you have an appointment with your friend, he told you that "I will be waiting for you between 15:30 -16:00,after 16:00 I'll leave the place".
If you arrive there at 15:45 you are in time
But if the appointment time is exactly 16:00 and
if you arrive there at 16:00 you are on time
Briefly for in time there is a time period but for on time there is an exact time.
Not a teacher.
Interested in Language